The oscillation of elevator cables, that is, cables having one end attached to an elevator car or to an elevator counterweight and being movable therewith, has been an ongoing problem for many years. Cable oscillations may be induced by the swaying motion of the structure, such as caused by wind, earthquake or other natural forces. Energy inputs particularly apt to induce cable oscillation are those where the energy input produces a lateral effect on the cable at or near one or more of the natural frequencies of oscillation of the cable. Cable oscillation may also be induced by air flow within the elevator shaft, such as that caused by stack action or elevator car movement. The motion of the elevator car itself may also contribute to elevator cable oscillation as the cable travels along with the car. Wind and earthquake-induced building oscillation can also be severely detrimental, leading to impacting of elevator cables against shaft walls, tangling of cables and the like.
All elevator cable systems, including the hoisting, compensating and electrical cables, can have oscillations induced with building motion and can develop resonance. The most severe difficulties are commonly encountered under the following conditions:
1) When building motion is induced by the wind or earthquake; PA1 2) When the elevator is stopped at a floor wherein the natural frequency of lateral oscillation of the cable coincides nearly coincides with the natural frequency of lateral oscillation of the building, often where the cable length is at or near a maximum; PA1 3) While the car is stopped and oscillatory motion is coupled from the building into the cables for some period of time; and PA1 4) When the elevator moves toward the other end of the cable, thus tending to shorten the cable, while oscillations are present in the cable. PA1 a) the increased wind-induced dynamic oscillation of taller buildings; PA1 b) the increased elevator speeds in taller buildings and PA1 c) the tendency toward resonance of elevator cables with the natural period of taller buildings.
For the compensating cables and suspended utility cables such as control, power and communication cables, the most severe case generally occurs after the car has been parked at an upper floor, where the cable is at its maximum unsupported length, for some period of time. During subsequent descent of the elevator, oscillation coupled from the building into the cables is increased in frequency of vibration and sometimes amplified in amplitude by the continuous shortening of the cable, much as the oscillation of a violin string is amplified as the finger moves down the bridge.
It can be shown that the swaying motions of tall buildings and the swaying modes of elevator cables within these buildings fall at or near one or more of the natural frequencies of oscillation of both building and cable.
The period of oscillation of a tall building (fundamental approximately equal to N/10, where N is the number mode) is very of stories in the building. For more or less constant tension and weight, the period of oscillation of a free cable is proportional to some function of its length. For hoist cables and the like (i.e., cables suspended from above and supporting a load such as an elevator car or counterweight) the natural frequency of oscillation takes the form: ##EQU1##
For compensating cables, electrical cables and the like (i.e., cables suspended at each end), the frequencies are more complex but are of the form: ##EQU2##
From these expression it can be seen that by providing temporary lateral support to elevator cables (i.e., moving member 40 and/or member 70 to its extended position), at intermediate points of the shaft, the free vibration length of the cable can be effectively significantly reduced. For cables laterally supported at both ends, a support near the midpoint will double the natural frequency of the cable, supports near the third points triple the natural frequency of the cable and so forth.
The problems of cable oscillation tend to worsen as building height increases. This is because of:
Various approaches to ease the problem of cable oscillation in elevator cables have been proposed. In one system, described in U.S. Pat. No. 1,145,914, excessive oscillation of a suspended electrical cable for an elevator is prevented by means of a stationary wire stretched vertically to one side of the path of the elevator between the bottom of the elevator and the top of the side wall of the shaft. This system cannot, however, be used to limit oscillation of the compensation and/or hoisting cables for the elevator. This system is also limited in oscillation can only be prevented in a side to side direction and only at the bight of the cable. Accordingly, this system is not readily adaptable to modern high rise structures.
In another proposal, described in U.S. Pat. No. 3,666,051, a horizontal guide member through which cables of the elevator pass is supported at an intermediate vertical location by stops on tracks on either side of the path of the elevator. When the elevator reaches the guide as it is raised, the elevator picks up the guide and causes it to be raised therewith. This system is disadvantageous in that noise and thumping can occur when the elevator reaches the guide, which can be disconcerting to passengers.
In another system, a dynamic damper consisting of an offset weighted bar is attached to the hoisting cables of the elevator near the elevator. This is said to cause lateral oscillations o the cable to be converted to twisting motions. This system, however, does not primarily limit the oscillations, but rather causes the oscillations to be damped once the have occurred. This system also apparently damps the motion of the cables at least in part by internal friction within the cables themselves, which can increase cable wear. Furthermore, the system is not readily adaptable to the suspended cables.
In another system, described in U.S. Pat. No. 4,117,908, oscillation of hoisting cables is limited by fixed guides positioned near the top of the shaft. However, because these guides are located near the top of the elevator path, to accommodate the guides, the elevator shaft must be built somewhat higher than would otherwise be necessary. Furthermore, because the guides can only practically extend a small portion of the length of the shaft, the effectiveness of this system in limiting oscillations of the cable at the midpoint of the cable is limited.
Various other approaches have been tried, including various damping systems, the use of traveling cars, the slowing of elevator cars, and programs for controlling the parking or continuous motion of elevator cars at levels within the elevator shaft so as to minimize the buildup of resonance-induced oscillator energy within the elevator cables. None, however, has proven entirely successful in limiting oscillations of elevator cables in modern high rise structures, except at very high cost.